Activated carbon is currently manufactured from a number of different sources like coconut-shell, coal, wood, etc., although coconut-shell is favored for high-end applications like electric double layer capacitors (EDLC). The main processes involved in the production of activated carbon from natural sources are: i) making the char (using suitable starting materials), ii) carbonizing the char; ii) removing impurities by etching/washing, and iii) activating. Activation may be done using a number of methods including high temperature treatment under carbon dioxide (CO2), oxygen (O2) or steam (the activating media in these cases), although steam activation is the most popular technique. In this process, steam at around 130° C. is flowed through a fluidized bed of carbonized char particles, between temperatures of around 800° C. and around 1,000° C. This process primarily results in micro-pores (<2 nanometers (nm)) when used with relatively hard starting materials like coconut-shell. CO2 activation is also a popular technique to produce porosity in different carbonized chars, although it is somewhat less efficient than steam activation (similar levels of activation require the CO2 process to operate at ˜100° C. higher than the steam activation process). Typical coconut-shell based activated carbon in commercial use today for EDLCs has a specific surface area around 1600 m2/gm, densities in the 0.4 gm/cc range, pore sizes of <2 nm and pore volumes around 0.7 cm3/gm (e.g. for the YP-50 brand of Kuraray Chemical Co., Japan).
Chemical activation of carbon has also been used to make activated carbon from different precursor materials. With this process, in its most basic embodiment, the carbonized material—in powder form—is mixed with certain chemicals and heated. The chemicals can be a) strong bases (most common: potassium hydroxide (KOH), sodium hydroxide (NaOH)), b) acids (most common: phosphoric acid (H3PO4)], or c) salts [most common: zinc chloride (ZnCl2)). Heating is typically performed at temperatures between ˜400° C. and ˜900° C., and depending on the starting materials, may also be carbonizing or pyrolyzing at the same time (when char is used instead of carbonized material).
Chemical activation has been shown to be effective in getting large surface-area carbons. For example, KOH activation of char from cellulose, potato starch and eucalyptus wood sawdust, have shown surface areas between 2000 m2/gm and 3000 m2/gm, when heating a mixture of dry powders of the char and KOH between 700° C. and 800° C. for 1 hour [L. Wei, et. al., 2011]. KOH activation of chars produced from sucrose resulted in a surface area of 2520 m2/gm when heated to 815° C. with a KOH to char weight ratio of 4.3 to 1 [Evans, M. J. B. et al, 1999]. Other studies of KOH treatment resulted in 3000 m2/gm after 600° C.-900° C. activation of a mixture of KOH and petroleum coke under inert atmospheres [Otowa, T. et al., 1996]. In another example that uses KOH activation (U.S. Pat. No. 8,927,103), a combination of a carbon starting material (pitch coke), KOH and polyethylene glycol was mixed thoroughly and formed into briquettes, which were activated by heating to 850° C. under atmosphere. Once cooled, it was washed extensively with sulfuric acid and water to ensure complete removal of the excess KOH and other reaction by-products.
Despite the high surface area of carbon obtained with KOH activation, this process is only in limited commercial use today—due primarily to its higher manufacturing cost compared to steam activation. A detailed examination of the KOH activation processes referenced above shows the following characteristics:    a. The ratio of KOH to carbon (by weight) is typically 3:1 to 4:1. This means that for every Kg of carbonized starting material, an additional cost of ˜4 times the cost of a Kg of KOH has to be added;    b. The potassium-containing by-products from the process have to be washed thoroughly (with water, solvents or acids) and disposed of, adding cost;    c. Finally, special handling and equipment are required due to the corrosive nature of KOH.
The mechanisms of KOH activation include the formation of metallic K above 700° C. [Wang, J. et al., 2012], which is highly reactive and needs to be appropriately handled.
Consequently, there exists a need for a simpler process that does not utilize harsh chemicals like the strong bases described above (e.g. KOH), that needs significantly lesser quantities of activating materials and does not require special handling equipment.
Chemical activation has also been done using salts like ZnCl2. U.S. Pat. No. 5,039,651 outlines a process for producing different shapes of activated carbon (disks, flats, etc.) by mixing the carbon precursor in powder form with an aqueous solution of zinc chloride. The ratio of the carbonized material to the dry weight of ZnCl2 varies between 1 to 0.6 and 1 to 3, although the highest surface areas obtained with this method were ˜1400 m2/gm (corresponding to the 1 to 1.25 ratio of carbonized material to ZnCl2). The carbon precursors here may be coconut shell, wood chips, saw dust, etc. Another example of ZnCl2 activation uses cherry pits as the carbon source, and resulted in 1971 m2/gm when the amount of ZnCl2 used was 5 times that of the carbon precursors [Olivares-Marin, M. et al., 2006]. Additionally, similar to the KOH activation processes (e.g. Evans, M. J. B. et al, 1999), the large amount of excess material needs to be washed, removed and disposed of appropriately, resulting in added costs to the manufacturing process.
Acids have also been used for chemical activation. Phosphoric acid solutions with concentrations of 80% resulted in activated carbon from olive pit precursors with a maximum surface area of ˜1200 m2/gm after 4 hours at 500° C. [Yakout, S. M., et al. 2012]. Assuming linear scale up from the methods described by Yakout et al., the volume of phosphoric acid required to produce 1 kg of activated carbon in an industrial scale setting is around 12 liters. H3PO4 activation of corncob resulted in a maximum of ˜700 m2/gin when pyrolyzed at 400° C. [El-Sayed, G. O., et al., 2014]. When wood was used as the precursor material, H3PO4 activation was found to give the best results of 1780 m2/gm when activated at 440° C. [Benaddi, H. et al., 2000] but even after removal of the excess phosphoric acid, chemical analysis showed about 0.4% phosphorous remaining. This level of impurity is not suitable for high-end applications like EDLC electrodes, which need phosphorus levels to be below 50 ppm. In all these examples, the amount of acid required was up to 5 times the weight of the carbon precursor materials. Overall, the use of acids as chemical activating agents is in limited industrial use due to the added costs associated with the material, handling systems and waste disposal.
While chemical activation methods do have the potential to create high surface area activated carbons, improvements in the process to lower costs and make the process simpler and safer, are desired.